st: generalized tobit model

dear statalist
I am trying to estimate a generalized tobit model
with two equations. The first equation is a probit
equation which determines whether a firm undertakes
R&D or not.
y1*=x1*b1 +u1 where y1* is a latent variable and the
firm undertakes research if y1*>0.
The second equation is a tobit equation which
determines the intensity of research if the firm
actually does research.
y2*=x2*b2 +u2 where y2*=y2(y2=actual observed
research intensity) if y1*>0.
The errors u1 and u2 are correlated(jointly normally
distributed).
Two questions
1)Is there a program/command in Stata which allows me
to estimate the above equations and the covariance
matrix in the simplest way without using the ML
command?
2)If ML command has to be used does anyone know how to
do it?
Any suggestion and help is extremely welcome. Thank
you.
Thoihen
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